Oil and gas drilling is a complex process in which a variety of different types of equipment is used. Generally, once a well bore is drilled to a pre-set depth into the earth, a casing, typically a cylindrical tube, is run into the well bore and cemented in place. This provides structural stability to the well and permits the operator to selectively produce and treat only certain zones. To ensure that the casing is centered in the well bore, so that it does not lean against the wall of the well bore, a device called a centralizer is positioned around the casing. The purpose of centralizers is at least two-fold. First, while the casing string is being run into the hole, the centralizers reduce the torque and drag factors seen by the operator while getting the casing to depth (some are designed to allow the casing string to rotate independently to further reduce friction while running in the hole). Another purpose is to center (or centralize) the casing once it has been run to total depth, so that when the cement is pumped, it can circulate all around the casing string to provide a good cement job. Typically, this means that cement has successfully been placed 360 degrees around the casing without a noticeable “wide” or “narrow” side to the annulus. Centralizers are typically formed as hollow-cylindrical tubes, although other types of centralizer geometries are also known. Once the casing is in place and is centered in the well bore with the centralizer, cement is pumped around the outer surface of the casing, between the outer surface of the casing and the wall of the well bore, in order to seal the well bore and to structurally support the casing. Once total depth is reached and all casing strings are cemented in the hole, the well can be selectively completed to allow oil to be extracted through the casing in a controlled manner.
Use of a centralizer in this process is important because, if the casing is not centered in the well bore, the annular cement layer will not form a strong bond in the area where the casing makes contact with the wall of the well bore, thus reducing the mechanical integrity of the well and reducing the proficiency of zonal isolation between formations. Conventional centralizers come in a variety of types, including solid centralizers which are formed of a hollow, cylindrical body having longitudinal blades on an outer surface extending along a length thereof. The blades can be solid, or spring-like blades. Solid centralizers are typically formed of metal or polymer materials. The spring-like blades are typically made of a metal material. Conventional steel or aluminum centralizers work well for their intended purpose, but they are heavy, difficult to handle, and do not provide a low coefficient of friction between the centralizer and the well wall (either casing, if there is casing already set in the upper portion of the well, or formation) thus causing the centralizers to experience high torque and drag forces. Polymer-based centralizers have become an attractive alternative to metal or alloy-based centralizers to alleviate some of the problems above. Use of a polymer material greatly reduces the coefficient of friction, thus reducing the torque and drag forces to which the centralizers are objected. However, polymer-based centralizers have their own disadvantages, in that they are worn down more easily by the abrasive forces of running in hole due to their lower mechanical strength. As a result, they can be worn down completely in some cases, thereby eliminating the centralizing function and potentially causing poor cement jobs where the casing string lays directly against the formation, thus increasing the health, safety, and environmental concerns, since the zones in the formation may not be effectively isolated.
Some attempts have been made to include projections on an outer surface of the centralizer, particularly the centralizer blades, to reduce the frictional forces exerted on the centralizer. For example, metallic centralizers have been formed with Teflon™ projections on an outer surface of the centralizer in order to reduce torque and drag forces while running downhole. However, such projections are sacrificial, i.e. wear down quickly, as the mechanism to reduce frictional forces, thus reducing torque and drag on the centralizer. That can be effective when the underlying blades are made of metal so as not to be sacrificed themselves, but in the situation where the underlying blade is a polymer material, the blade itself would also be quickly sacrificed, causing the problems outlined above.
What is needed is a solution for polymer blades to reduce wear and abrasion without sacrificing the wear reducers and subsequently the polymer blades themselves.